Process for the high concentration of nitric acid



Aug. 5, 1930. I F. c. ZEISBERG 1,772,122

PROCESS FOR THE HIGH CONCENTRATION OF NITRIC ACID Fi led June 27, 1927 I 1 Y SIROHG REILUX SULFURIC ACID C K CONDENSER l w J Sfronq flii'ric Acid DEHYDRATIHG. V

OWEIL T /A Weak flifric Acid DEI1ITRATING Sream J G ,Dlluhz nnrn; 001C} Weak Sulfuric Acid HedCZebbe/y lnvznror 51 his ahbmeq Mama Patented Au 5 1930 uulrno STATES PATENT OFFICE FRED C. ZEISBERG, OF WILMINGTON, DELAWARE, ASSIGNOR TO E.- I. DU PONT DE NEMOURS &; COMPANY, OF WILMINGTON, DELAWARE, A CORPORATION OF DELA- WARE Application filed June 27,

This invention relates to the concentration of nitric acid to a high strength by means of a dehydrating agent and heat, with a minimum use of such dehydrating agent.

It is well known that nitric acid of an intermediate strength, say from 50% to 70% HNO is most economically concentrated to a high strength by mixing with it a dehydrating agent to bind or hold back the water and then applying heat to the mixture to distil out the nitric acid. Many such dehydrating agents have been proposed, but the one which has found the most favor indus trially is, overwhelmingly, concentrated sulfuric acid.

In the oldest and best-known process using sulfuric acid, the mixtureof dilute nitrio acid and concentrated sulfuric acid is put ina retort and heat is applied. The nitric acid distils out, the first portion coming over strong, but as the distillation proas to cause excessive action on the castiron retorts customarily used in the process, require that a comparatively large amount of sulfuric acid be used in the process. Assuming'that 60% nitric acid is to be concentrated by means of 92% sulfuric acid used in such proportions that the residual acid from the retort contains not less than 80% H SO from 3.7 5' to 4.00 pounds of H SO must be used for every pound of HNO concentrated.

This sulfuric acid, in a self-contained proc ess, must be concentrated to its original strength for reuse, and ,this concentration constitutes the principal item of expense in the whole process. Moreover, the usual ob jections to a batch process, as opposed to one operating continuously, obtain as well.

To overcome intermittent operation and 1927. Serial No. aoneoa avoid the repair costs incident to the action of the acid on the cast iron of the retorts used in thedistillation, a continuous procbut in every case the heating medium is steam which condenses into the acid mixture.

Steam, when used in this Way, while it is p a convenient source of heat, possesses the decided disadvantage that the water resulting from its condensation dilutes the sulfuric acid used in the process, and must be boiled out of the sulfuric acid before this acid can be used again; Again assuming 60% nitric acid concentrated by means of 92% sulfuric acid, for every pound of HNO concentrated 0.614'poundof water which originally accompanied the HNO -must beboiled out of the sulfuric acid, while from 0.75 to 0.90 pound of water resultingfrom the condensation of the steam must be boiled out. The condensed steamtherefore exceeds the original diluting water. Moreover, largely because of this added water, it is necessar to concentrate a minimum of 4.0 pounds Hi0. for every pound of HNO concentrated.

As opposed to this extensive concentration of sulfuric acid in the prior art, I have found that by my novel process it is necessary to concentrate only about 2.75 pounds H SO per pound HNO when starting with 60% nitric acid and 92% sulfuric acid, and that in addition to the water originally present in the nitric. acid, it is necessary-to boil out ofthis sulfuric acid only about 0.08 pound water resulting from the condensation of steam, a saving over the- Pauling process of 1.25 pounds H 80 and 0.057 to 0.82 pound water for every pound HNO concentrated. My process consists in passing vapors of condensate is drawn out of the process and run to storage. The hot liquid acid mixture running out of the bottom of the dehydrating tower is run into a denitrating tower, descriptions of which abound in the prior art, and steam is introduced into the bottom of this tower in only enough amount substantially to denitrate the mixture which emerges from the bottom of the dehydrating tower,

I the nitric acid escaping from the top of the denitrating tower as vapors having substantially the. same composition as the vaporsoriginally introduced into the bottom of the dehydrating tower; These vapors from the denitration are joined to those entering the dehydrating tower. Any oxides of nitrogen which may be formed in the process'pass through .the reflux condenser uncondensed and may be converted to weak nitric acid n absorption towers customarily used for this purpose.

The drawing shows a diagrammatic side elevation ofone assembly of apparatus suitable for carrying out my process. A is the dehydrating tower, constructed of acid-proof material and packed with some suitable packing, such as quartz. B is a storage tank for concentrated sulfuric acid. By means through of a connecting line carrying a valve 0 this acid is continuously and uniformly introduced into A. Into the bottom of A. is in- 'troduced a stream of nitric acid vapor generated in'the boiling pot D, which may confsist of a chrome iron pot containing a steam coil, though direct fire or any other controllablesource of heat may equally'well be used.

The weak nitric acid to be concentrated is introduced into the boi ling pot D through the valved line; E; Itsrate of addltionand the rate of heating are so controlled that with a constantsu pply of strong sulfuric acid from B, the denitrated 'acid emerging from the trap H has a"'strength of about 73% H SO The vapors of highlyf eoncentrated nitric acid leaving Alpa'ss through a line to the reflux condenser I and are there condensed to a liquid which runs out through valved line J. This valve is partly closedlso-that the' condensatbacks up, and part of'it flows back through the line K to the top of the dehydrating; tower A. The part .passing 'ne J goes to thestrong nitric acid storage (not shown). The amount refluxed varies with its temperature and the temperature of the sulfuric acid. If their average temperature is about 20 0. about 1.5 to 1.7 5 pounds are returned for every pound taken out of the system.

The hot sulfuric acid leaving A contains practically all of the water of the nitric acid vapors from D, and a few percent of nitric acid and is in equilibrium with the vapors entering the bottom of the dehydrating tower. It enters the denitrating tower G through the trap F. Ur is the usual packed acid-proof tower of the denitrating art and in it the acid mixture is denitrated by means of steam controllably introduced through line L. The substantially completely denitrated sulfuric acid is run out at H and goes to the sulfuric acid concentration plant, whence it is returned to B for reuse. Only a very small amount of steam needbe introduced-through line L, usually from 5 to 10% of the weight of nitric acid concentrated, the rate of its introduction being governed by the strength of the vapors leaving tower G. These are maintained at a strength but little, if any, below the strength of the vapors from pot I); if they are much lower less steam is introduced through line L. The nitric acid vapors leayxing G are introduced into the bottom of Instead of refluxing the nitric acid condensate the same result can be achieved by precooling the dehydrating agent before introducing it into the dehydrating tower, but

practically it is easier to reflux the nitric acid.

If the reflux is cut down, or the dehydrating agent is warmed I have found that the quancity of dehydrating agent necessary to accomplish a certain amount of concentration is increased, while if the reflux is too greatly increased or too great a cooling effect is produced at the top of the dehydrating tower the acid mixture leaving this tower will be difficult to denitrate. In general, I prefer to use such an amount of reflux that the acid mixture entering the d'enitrating tower contains not more than 2 or 3% HNO and if such is the case the amount of H 80 required per pound of HNO concentrated will not exceed 3 pounds when starting with 60% nitric acid and 92% sulfuric acid.

It might be thought that the process described by Pauling in U. S. Patent 993,868 is analogous to my invention, but that Pauling did not understand the principles of my invention is shown by his direction to preheat the sulfuric acid introduced to his dehydratmg columns, whereas I prefer to cool the sulfuric acid in my process.

I have described one modification of my process, but it is obvious to one skilled in the art that there are other ways in Which it might be carried out. Thus, it is not necessary to use towers, any form of countercur- '60 opposite end of said countercurrent a rent apparatus being suitable; or, various methods of vaporizing the nitric acid might be used. Or, if a use for very dilute nitric acid existed, the vapors from the denitrating tower could be sent to a separate condenser instead of to the foot of the dehydrat ing tower, in which case the amount of S111- furic acid used to concentrate a pound of HNO might be reduced to 2 pounds or less. If desired, the two towers might be combined in one. All or any of thesemodifications might be carried out without departing I of said counter-current apparatus by any suitable means, subjecting the escaping hot nitric-acid containing dehydrating agent to just sufficient steam to completely denitratc it, and passing the resulting nitric acid vapors into the first mentioned countercurrent apparatus along with the original vapors of dilute nitric acid. f

3. A process of producing highly concensaid nitric acid vapors, thereby condensing and returning the water contained therein to the countercurrent system; subjecting the escaping hot sulfuric acid to denitration with steam to produce approximately-% nitric acid vapor; passing the resulting nitric acid vapors into said dehydrating tower; the introduction of the 60% nitric acid vapors and the conditions of operation being so controlled that the denitrated sulfuric acid wi l have a strength of about 73% H 80 In testimony whereof I aflix my signature.

FRED C. ZEISBERG.

trated nitric acid which comprises introduc-.

ing vapors of dilute nitric acid into one end of a dehydrating tower, introducing an unheated countercurrent of concentrated sulphuric" acid into: the extreme opposite end of said tower, refluxing and returning to said extremeopposite end of the tower a substantial portion ofthe concentrated nitric acid evolved, and withdrawing hot dilute nitric acid-containing sulphuric. acid from the end of said dehydrating tower at which the original nitric acid vapors are introduced.

4. The process of claim 3 wherein the es caping 'hot nitric acid-containing sulfuric acid is subjected to just suflicient steam to .completely denitrate it, and the resulting nitric vapors are passed into the tower with i the original vapors of dilute nitric acid. 5., A. process'of producing highly concentrated nitric "acid by introducing vapors of w about 60% nitric acid at one end of a countercurrent of about 9 2% sulfuric acid in a de hydrating tower; refluxing at the extreme portion of the nitric acid vapors evo ved equivalent on the basis. ofa. temperature of 20 C. for incoming sulfuric-acid, to about 4.5 to 1.75 pounds perpound nitric acid removed from the system to abstractheat'from 

